The circulating component of the mammalian circulatory system comprises various cell types, including red and white blood cells of the erythroid and myeloid cell lineages. See, e.g., Rapaport (1987) Introduction to Hematology (2d ed.) Lippincott, Philadelphia, Pa.; Jandl (1987) Blood: Textbook of Hematology, Little, Brown and Co., Boston, Mass.; and Paul (ed.) (1993) Fundamental Immunology (3d ed.) Raven Press, N.Y.
For some time, it has been known that the mammalian immune response is based on a series of complex cellular interactions, called the “immune network.” Recent research has provided new insights into the inner workings of this network. While it remains clear that much of the response does, in fact, revolve around the network-like interactions of lymphocytes, macrophages, granulocytes, and other cells, immunologists now generally hold the opinion that soluble proteins, known as lymphokines, cytokines, or monokines, playa critical role in controlling these cellular interactions. Thus, there is considerable interest in the isolation, characterization, and mechanisms of action of cell modulatory factors, an understanding of which should lead to significant advancements in the diagnosis and therapy of numerous medical abnormalities, e.g., immune system and other disorders.
Lymphokines apparently mediate cellular activities in a variety of ways. They have been shown to support the proliferation, growth, and differentiation of the pluripotential hematopoietic stem cells into vast numbers of progenitors comprising diverse cellular lineages making up a complex immune system. These interactions between the cellular components are necessary for a healthy immune response. These different cellular lineages often respond in a different manner when lymphokines are administered in conjunction with other agents.
The chemokines are a large and diverse superfamily of proteins. The superfamily is subdivided into two classical branches, based upon whether the first two cysteines in the chemokine motif are adjacent (termed the “C-C” branch), or spaced by an intervening residue (“C-X-C”). A more recently identified branch of chemokines lacks two cysteines in the corresponding motif, and is represented by the chemokines known as lymphotactins. Another recently identified branch has three intervening residues between the two cysteines, e.g., CX3C chemokines. See, e.g., Schall and Bacon (1994) Current Opinion in Immunology 6:865-873; and Bacon and Schall (1996) Int. Arch. Allergy & Immunol. 109:97-109.
The chemokine receptors are typically members of the superfamily of G-protein coupled (or linked) receptors (GPCR, or GPLR). As a class, these receptors are integral membrane proteins characterized by amino acid sequences which contain seven hydrophobic domains. See, e.g., Ruffolo and Hollinger (eds. 1995) G-Protein Coupled Transmembrane Signaling Mechanisms CRC Press, Boca Raton, Fla.; Watson and Arkinstall (1994) The G-Protein Linked Receptor FactsBook Academic Press, San Diego, Calif.; Peroutka (ed. 1994) G Protein-Coupled Receptors CRC Press, Boca Raton, Fla.; Houslay and Milligan (1990) G-Proteins as Mediators of Cellular Signaling Processes Wiley and Sons, New York, N.Y.; and Dohlman, et al. (1991) Ann. Rev. Biochem. 60:653-688. These hydrophobic domains are predicted to represent transmembrane spanning regions of the proteins. These GPCRs are found in a wide range of organisms and are typically involved in the transmission of signals to the interior of the cell, e.g., through interaction, e.g., with heterotrimeric G-proteins. They respond to a wide and diverse range of agents including lipid analogs, amino acid derivatives, small peptides, and other molecules.
The presumed transmembrane segments are typically 20-25 amino acids in length. Based upon models and data on bacteriorhodopsin, these regions are predicted to be α-helices and be oriented to form a ligand binding pocket. See, e.g., Findley, et al. (1990) Trends Pharmacol. Sci. 11:492-499. Other data suggest that the amino termini of the proteins are extracellular, and the carboxy termini are intracellular. See, e.g., Lodish, et al. (1995) Molecular Cell Biology 3d ed., Scientific American, New York; and Watson and Arkinstall (1994) The G-Protein Linked Receptor FactsBook Academic Press, San Diego, Calif. Phosphorylation cascades have been implicated in the signal transduction pathway of these receptors.
Although the full spectrum of biological activities mediated by these 7 transmembrane receptors has not been fully determined, chemoattractant effects are recognized. Chemokine receptors are notable members of the GPCR family. See, e.g., Samson, et al. (1996) Biochemistry 35:3362-3367; and Rapport, et al. (1996) J. Leukocyte Biology 59:18-23. The best known biological functions of these chemokine molecules relate to chemoattraction of leukocytes. However, new chemokines and receptors are being discovered, and their biological effects on the various cells responsible for immunological responses are topics of continued study.
Many factors have been identified which influence the differentiation process of precursor cells, or regulate the physiology or migration properties of specific cell types. These observations indicate that other factors exist whose functions in immune function were heretofore unrecognized. These factors provide for biological activities whose spectra of effects may be distinct from known differentiation or activation factors. The absence of knowledge about the structural, biological, and physiological properties of the regulatory factors which regulate cell physiology in vivo prevents the modulation of the effects of such factors.
In addition, other factors exist whose functions in hematopoiesis, neural function, immune development, and leukocyte trafficking were heretofore unrecognized. These receptors mediate biological activities whose spectra of effects are distinct from known differentiation, activation, or other signaling factors. The absence of knowledge about the structural, biological, and physiological properties of the receptors which regulate cell physiology, development, or function prevents the modification of the effects of such factors.
Thus, medical conditions where regulation of the development or physiology of relevant cells is required remain unmanageable.